This invention relates to a method and apparatus for recording and reproducing periodic information signals, such as composite color video signals, and more particularly, is directed to a method and apparatus for recording and reproducing color video signals wherein information relating to different colors is arranged in line-sequence, and wherein such a line-sequential color information signal is recorded in color alignment.
In a typical video recording system, such as a video tape recorder (VTR) which is capable of recording composite color video signals with high recording density, the color signals are recorded in successive, adjacent record tracks which are provided without guard bands therebetween. In this format, the record medium is utilized efficiently because "blank" portions thereof need not be provided between adjacent record tracks. To minimize crosstalk interference when video signals which are recorded in this format are reproduced, the incoming video signal is processed prior to the recording thereof. Typically, the luminance and chrominance components of the composite color video signal are separated from each other, the luminance component is frequency-modulated to a relatively higher frequency band and the chrominance component is frequency-converted down to a relatively lower frequency band. Then, the processed luminance and chrominance components are combined for recording in successive parallel tracks. Usually, a single field of the video signal is recorded in each track, thereby resulting in the recording of a frame in two successive tracks. In one type of VTR, a pair of magnetic transducers, or recording heads, rotatably scan the magnetic tape, each head being supplied with a single field so as to record that field of video signals in the track which is scanned thereby. In order to reduce crosstalk interference, the air gaps of the two heads are provided with different azimuth angles. Thus, one field, such as the odd field, in each track is recorded with one azimuth angle while the other field is recorded with a different azimuth angle.
During reproduction, the transducers, or playback heads, which are used to reproduce the recorded video signals are provided with the same azimuth angles as were used during recording. Since there are no guard bands to separate adjacent tracks, it is likely that, when one playback head scans its appropriate track to reproduce the video signals recorded therein, it also will pick up a crosstalk component from the adjacent track. However, because of the phenomenon of azimuth loss, since the picked up crosstalk component had been recorded with a different azimuth angle, the picked up crosstalk component will be reproduced with substantial attenuation. Azimuth loss is directly related to the frequency of the recorded signal, so that the reproduced luminance crosstalk component, which had been frequency modulated to a relatively higher frequency band, will be seriously attenuated.
The aforementioned phenomenon of azimuth loss is not as effective in minimizing chrominance crosstalk components. This is because the chrominance components had been frequency-converted down to a relatively lower frequency band during recording. Accordingly, in order to reduce crosstalk interference, adjacent tracks are recorded in so-called H-alignment; that is, the horizontal synchronizing intervals in each track are aligned transversely across the tracks. This H-alignment occurs if the distance that the tape moves during the recording of one field interval is equal to a whole number of lines plus half a line so as to account for the phase at the start of the next field.
Although the foregoing technique generally is used to record color video signals which are present in various formats, such as the NTSC, PAL and SECAM formats, a particular problem may arise if the color video signal is in a line-sequential format, such as the SECAM format. As referred to herein, a line sequential color video signal is of the type wherein successive line intervals are provided with color information signals which relate to different colors. For example, odd line intervals may include blue color information while even line intervals may include red color information. In the SECAM format, this line sequential color information is provided by frequency-modulating a blue subcarrier of about 4.25 MHz with blue color difference signals (B-Y) followed by frequency-modulating a red subcarrier of about 4.41 MHz with red color difference signals (R-Y). Thus, the blue and red color information signals appear alternately. Furthermore, since the SECAM color video signal is provided with 625 line intervals in each frame, the color information which is recorded in the first line interval also is recorded in the last line interval of that frame. This means that, in odd frames, the blue color information may be provided in odd line intervals while the red color information may be provided in the even line intervals, while in the even frames, the blue color information may be provided in the even line intervals while the red color information may be provided in the odd line intervals. Of course, the converse of this also may occur.
When a SECAM color video signal of the aforementioned type is recorded in accordance with the technique discussed above, then, because the first field of one frame generally is shifted by 1.5H (H is the length or delay of a horizontal line interval) from the start of the last field of a preceding frame, successive frames are recorded in the absence of color alignment. That is, in one frame, constituted by two tracks each containing a single field, adjacent line intervals are provided with information relating to the same color. Hence, blue color information is aligned transversely of the record tracks, followed by aligned red color information, aligned blue color information, and so on. However, when the next frame is recorded, the blue color information which is present in the preceding frame is aligned with the red color information in the next following frame. A representation of this type of recording is illustrated in FIG. 1 of the accompanying drawings wherein the subscript 1 represents frame 1, or odd frames, and subscript 2 represents frame 2, or even frames. Furthermore, subscript a represents the first field in each frame while subscript b represents the second field in each frame. In FIG. 1, those line intervals which contain blue color information are illustrated without cross-hatching, and those line intervals which contain red color information are illustrated with cross-hatching.
From FIG. 1, it is seen that, in a frame, the beginning of the first field in that frame is displaced from the beginning of the following field by a distance equal to one-half a horizontal line interval, this displacement being in the direction of the track. Thus, the beginning of the track T.sub.a1 in which the first field is recorded is displaced from the beginning of track T.sub.b1 in which the following field is recorded by 0.5H. Also, the beginning of the first field in one frame is displaced from the beginning of the second field in the immediately preceding frame by one and one-half horizontal line intervals. That is, the beginning of track T.sub.a2 in which the first field of, for example, the second frame is recorded, is displaced from the beginning of track T.sub.b1 in which the second field of the first frame is recorded by 1.5H. Furthermore, in one frame, such as the frame recorded in tracks T.sub.a1 and T.sub.b1, the horizontal line intervals in these respective tracks are in color-alignment with each other. That is, red color information signals are recorded in adjacent line intervals, and blue color information signals are recorded in adjacent line intervals. However, although tracks T.sub.a1 and T.sub.b1 are recorded in color-alignment, and tracks T.sub.a2 and T.sub.b2 are recorded in color alignment, track T.sub.b1 is not in color-alignment with tracks T.sub.a2. That is, although the first and second fields of a given frame are recorded in color-alignment, adjacent frames are not recorded in such color-alignment.
The SECAM color video signal is formed of 625 line intervals. Each frame is constituted by 312.5 line intervals. For proper alignment of the horizontal synchronizing intervals, track T.sub.b1 is displaced by 0.5H from track T.sub.a1. If it is assumed that two rotary transducers are used to record the successive tracks, and if it is further assumed that these two heads, referred to for the purpose of the present discussion as heads A and B, are separated from each other by 180.degree., and if the tape is transported a distance equal to 1H for each pass of a head thereacross, then successive tracks will be displaced by an amount equal to 1H. In order to record the tracks in the format shown in FIG. 1, heads A and B must be separated from each other by an angle of 180.degree.-.alpha.. The angle .alpha. is such that the tape is transported by 0.5H from the time that head A scans the tape until head B reaches the tape; and the tape is transported by an amount equal to 1.5H from the time that head B scans the tape until head A reaches the tape.
When the SECAM color video signal is recorded in the format shown in FIG. 1, that is, wherein information relating to a particular color is recorded in odd line intervals in odd-numbered frames and in even line intervals in even-numbered frames, there is no difficulty in reproducing the original video signal during a normal playback operation. This is because the respective playback heads scan the same tracks during a reproducing operation as were scanned by such heads (or the equivalent thereof) during recording. That is, the scanning traces of the heads during a reproducing operation are substantially coincident with the scanning traces of the heads during a recording operation. However, there is a problem when the recorded SECAM color video signal is reproduced during a slow-motion or still-motion mode. In such modes, the speed at which the tape is transported is less than the normal recording/reproducing tape speed. As a consequence thereof, the playback heads do not scan the same traces during a slow-motion or still-motion reproducing operation as were scanned during a normal recording operation. As an example of the scanning trace of each playback head during such slow-motion or still-motion reproducing operations, reference is made to FIG. 1 wherein the trace of the playback head is illustrated by the broken line A. It is seen that this scanning trace crosses over a number of tracks. Furthermore, since the SECAM color video signal is not recorded in color alignment from one frame to the next, scanning trace A does not cross over alternate color information signals. Rather, when this scanning trace traverses adjacent tracks which are associated with different frames, for example, tracks T.sub.a2 and T.sub.b1, color signals relating to the same color are reproduced in successive line intervals. More particularly, as scanning trace A traverses tracks T.sub.b2, T.sub.a2, T.sub.b1 and T.sub.a1, in sequence, the color difference signals which are reproduced by the playback head are seen to be (B-Y), (R-Y), (R-Y) and (B-Y), respectively.
As a result of the scanning of two successive line intervals having color information signals therein relating to the same color, the line-sequential format of the SECAM video signal is disturbed. Consequently, if the video signals which are reproduced during the slow-motion or still-motion mode are supplied to a television monitor, the displayed video signal will exhibit substantial color noise, or interference. This is because, since the line-sequential arrangement is distorted, a (B-Y) color difference signal will be misinterpreted as a (R-Y) color difference signal in some line intervals and will be supplied to the red color demodulator. Similarly, in other line intervals, the reproduced (R-Y) color difference signal will be misinterpreted as the (B-Y) color difference signal and will be supplied to the blue color demodulator. Consequently, the respective color demodulators will not be capable of demodulating the particular color difference signals which are supplied thereto. Since color discrimination cannot be attained properly, the usual color killer circuit which is provided to avoid erroneous display of a color picture will be operated. Although the usual color discriminating signals are provided in each vertical blanking interval of the recorded SECAM color video signal, these discriminating signals, when reproduced, will establish a particular switching condition for switching the reproduced line intervals alternately to different ones of the color demodulators. But since the reproduced color information signals do not alternate properly, incorrect color difference signals will be supplied to the respective red and blue color demodulators. Consequently, the aforenoted problem of displayed color noise or of color killer operation is present.
In order to avoid this mis-operation of the color demodulators in a SECAM television receiver, the color information signals in all frames should be recorded in color-alignment. That is, in addition to the normal color-alignment of both fields in a given frame, there also should be color-alignment between frames. A type of color alignment is described in U.S. Pat. No. 3,852,520. However, as described therein, a given color information signal is recorded in, for example, odd line intervals in both odd and even frames. This means that if, for example, the (R-Y) color difference signal is recorded in the last, or 625th, line interval of one frame, it also is recorded in the first line interval of the next following frame. However, in a conventional SECAM color video signal, if the (R-Y) color difference signal is provided in the 625th line interval of one frame, then the (B-Y) color difference signal should be provided in the first line interval of the next-following frame. Consistent with this normal convention, a typical SECAM television receiver is adapted to respond to the first line interval of the next following frame as containing color information which is different from the last line interval of the immediately preceding frame. But, since the same color information signal is recorded in the last and first line intervals of all frames, in accordance with the technique described in U.S. Pat. No. 3,852,520, the video signals which are reproduced are not in proper line-sequence from one frame to the next. Thus, the aforementioned difficulty in color noise and color killer operation will be present when the technique described in this patent is used for the recording and reproducing of SECAM color video signals in color-alignment.